How I can extrude down?
..Hard to reverse-engineer what you've done, but there are a couple of clues in your picture:

The edges in the top surface look a little thick? There are possibly some doubled faces / vertices there. Try X deleting, or H hiding some of the faces, to see if there are more hidden underneath. It's a feature of Blender's extrude, that if you cancel by right-clicking, you don't cancel the extrusion. You cancel the move, so the generated faces remain, exactly over the originals.
The Gizmo indicates that something is being operated on, halfway down the bottle. AltZ go into X-Ray mode, and see if there's a face there, which is the one being extruded, out of sight.